During the signal collection process used in direction finding (DF), signals are processed by multichannel receivers which take the collected signals and coherently frequency shift them down in frequency to an Intermediate Frequency (IF) or to baseband, so they can be digitized and processed at a lower frequency that is adequate for high bit Analog to Digital Converters (ADCs).
In order to perform the direction finding operation, the cross product of multiple receiver channels is taken to obtain the cross phase between each two antenna components or antenna pairs. For example, in a 3 channel coherent receiver, the cross phase is obtained between channels, 1 and 2, 2 and 3, and 1 and 3. Direction finding accuracy is often compromised when using most multi-channel receivers since the frequency shift operation results in phase and/or amplitude distortions of each signal, that add to perturb the true cross product of the antennas, and therefore perturb the effective cross phase. This distortion is often caused by temperature, time, and vibration instability of the various RF and IF components in the receivers, which in effects adds colored noise into the cross phase outputs.
The conventional solution to the accuracy problem in direction finding is to use a switched calibration (or boresight) method. This process involves injecting a calibration (or boresight) signal into the path between the antennas and the multichannel receiver. However, this method has serious shortcomings. In the switched method, the antennas are momentarily switched out, when the calibration source is switched in, This occurs in all input paths. All calibration inputs are coherent since they originate from a split (power) copy of the calibration signal. This calibration signal is then down-converted in the receiver and sampled, using a coherent sampling (timed) signal, for each independent receiver output.
Finally, this signal is processed to generate a calibration (or boresight) vector, used to compensate the collected far field signals. This switched boresighting process cannot occur at the same time as collecting the far field signals required for DF, since the DF antennas have been switched out. Additionally, this process requires the user to calibrate (boresight) the system many minutes or hours prior to actual DF operation. This is disadvantageous because the resulting boresight vector becomes old or no longer adequately represents the current amount of phase and gain variations or distortions. This method is also handicapped because the signal collection activity required in direction finding must cease and the reference signal must be emitted or “switched in” so the receivers can process the reference signal; therefore, calibration and collection of the desired signal cannot occur at the same time. That is, the wideband (noise) signal must be switched in, dwelled and collected, and then switched back out. There must be a switch back to the desired signal and antennas.
The result of this process is that the amount of time spent boresighting is limited. The switch in method does not permit simultaneous boresight/calibration signal collection and DF operation.